Mounted Combat/Useful Notes
Horses were ridden into battle for thousands of years, but most human beings today are unlikely to see one except at a recreational stable. This Useful Notes article exists to talk a little bit about mankind's favorite beast of burden, and how it performs in war.
Horses are not born fighters. Although warhorses have been bred over the centuries for calmness, strength, and stature, a horse's first instinct when facing an individual with a sharp piece of metal and a Slasher Smile is to flee as fast as it can. Fighting is not completely alien to the animal's nature; anything that gets too close to its rear can expect a nasty kick from both hind legs. However, the fact that its main weapon is rear-facing only reinforces the idea that a horse's first instinct when faced with a threat is to flee, fighting only to keep retreating if fleeing fails.
This is somewhat counterproductive to the nature of the animal that rides it; humans are rather more bellicose. (Well, some of them, anyway.) As a result, humans have tried to prepare their mounts for combat in various ways, starting by selecting horses with traits suited to the battlefield for breeding. Horses bred for trust in their rider, size, strength and, above all, a calm demeanor, quickly became the warhorses that carried the richest warriors into battle. Further training was carried out to prepare them to overcome their natural instinct to flee; horses in Roman times were prepared for battle by simulating the noise and clamour of battle so as to make the animal used to such alarming circumstances.
Once in battle, a man on horseback has several advantages over a man on foot. He is higher up, giving his attacks gravity-aided force and intimidating those he towers over. More crucially, he can move much faster, meaning he dictates the terms of any fight. For this reason, Archery was initially the favored discipline for those who rode into battle; the samurai didn't call their art "The Way Of Horse And Bow" for nothing. (The sword came later.) This maneuverability was not restricted to ranged fighting however; a common tactic of a cavalry soldier was to canter around an isolated infantryman in a tight circle hacking at him while the poor man on the ground had to keep turning and defending. Unless the horseman could be dismounted or the horse injured, the man on foot would inevitably lose eventually as a mixture of gravity and disorientation took its toll.
On the flip side, however, a horse is a big target. Archers on foot have a much bigger target to aim at than their mounted counterparts, and if the horse was felled, the horse's rider faced a painful tumble, after which still had to get up before some infantryman could finish him off. If the horse fell on its rider, it was over. Furthermore a horse is not as stable a fighting platform as solid ground, and riders were vulnerable to being toppled; the heavier the armour of the rider, the worse this was for him. Not to mention the number of accounts of wounded horses charging off the battlefield as their bemused rider was carried off the field with the terrified animal!
Horses, however, are generally used en-masse in battle. The horse began to supplant the chariot around the 9th century BC as Iranian tribes were recorded using bows in battle, but it wasn't until the invention of the saddle and later stirrups, both of which make it way easier to stay on the horse during maneuvers, that the cavalryman could fight in melee or on the charge with some degree of stability. Horsemen were initially used as fast-moving archers, evolving into flank-screening forces. It was Phillip of Macedon's military reforms that really turned the cavalryman into the shocking, battlewinning formations, the picturesque large body of cavalry slamming into disordered infantry and driving them off the field. His son Alexander would go on to win entire battles based around this use of cavalry as the hammer with which to break the enemy army upon an infantry anvil.
Cavalry remained relegated to judiciously timed flank assaults and archer support, however, until the coming of the late Roman period, when the Huns proved that heavy cavalry could beat heavy infantry. (This was one of the factors, incidentally, which sealed the still infantry-heavy Roman army's fate... though the Eastern Romans managed to adopt a much more cavalry-heavy army.) After the collapse of the Western Roman and Hunnic empires, the Normans initially were amongst the first mounted soldiers who had a decent chance of giving an infantry formation serious pause for thought from the front, breaking the dominance of the infantry soldier through Europe's dark ages, though they preferred to soften such formations up with hails of javelins from horseback and, when that failed, archer support, before punching through the weakened lines. As time wore on and armour (and possibly more crucially, the archer component) got better, this light javelin was replaced by the couched lance, the iconic arm of the medieval mounted knight.
This form of warfare reached its peak in the 13th century, with glorious depictions of massed knights charging over the enemy's foot; but cavalry were never able to just waltz over ordered and disciplined infantry, ones with pikes or other polearms in front and enough nerve to stand against the charge. The only time a heavy cavalry assault really worked was when the infantry lacked discipline or lost formation (usually thanks to already being engaged or, as the Scots found out to their cost, due to archer fire). Once the enemy formation had been broken, the knight was devastating, but that depended at least partially on the enemy, and charges were to a certain extent all-or-nothing propositions. For that reason, wise Medieval generals kept a solid body of disciplined infantry behind so that the cavalry charge, if it failed, had somewhere to fall back to, and the general a reserve force to send in; several battles were won by the infantry after the initial cavalry force had been driven from the field. In terms of formation, heavy cavalry would keep together as closely as possible, giving their horses nowhere to go but forward and the enemy in front nowhere to go but back or towards a relentless press of horseflesh, steel and aggression; trampling was certain either way.
The knight, however, was no match for the steppe warrior to the east; when Genghis and his Golden Horde swept out of the east the armoured knight was utterly smashed by the devastating composite bow and the incredibly disciplined and utterly ruthless Mongol cavalryman, capable of running circles around heavier opposition due to a combination of light-weight equipment and each Mongol having multiple, very durable ponies as opposed to a single large war horse. These smaller horses also had a very significant advantage of being able to live off the land--a giant European warhorse would starve to death if it tried to live only on grass, as its inefficient digestive system lagged behind its selectively bred size. All this allowed the Mongols to travel as much as 70 kilometers a day, faster than their enemies' information channels, often catching their targets completely off guard. Only with the arrival of gunpowder in Russia did the terrifying power of the steppe nomad begin to retreat, the Russians winning a decisive victory over the declining Mongol empire at Ugra in 1476.
The heavily-armoured knight began to decline in the 14th century and had more or less been replaced by the back-and-breast armoured, saber-wielding cavalryman by the turn of the 1600s. During the English Civil War the cavalryman was the bane of the infantry on the flanks; the side which gained cavalry dominance generally won the battle as the losing side's infantry were trampled from behind. (Unless the winning side's infantry had already conclusively decided the infantry battle, leaving the cavalry nothing to ride down.) Gone, however, were the days of cavalry being able to break up an infantry formation on their own; faced with disciplined pike and shot the cavalry couldn't do much more than harry the infantry with pistol shot. Also notable in this period was the development of the dragoon: infantry who rode into position before dismounting and fighting on foot. They were named after their distinctive firearms, but known for their speed.
Cavalry continued to develop as time wore on, but it never regained its supremacy. Cavalry could still be dangerous; during the Napoleonic Wars infantry needed to form a special square formation in order to prevent cavalry from overrunning their lines, and during the Crimean War incidents such as the Charge Of The Light Brigade showed that even when supposedy completely outclassed, the cavalryman could still achieve their objectives (despite the horrific losses the cavalry did actually manage to take the guns; the merely failed to hold them. British cavalry were scary, scary folk.) In the same battle (the Battle of Balaclava) however, the "thin red line" incident demonstrated just how vulnerable cavalry could be when taking on disciplined infantry directly; a mere regiment of British soldiers, the 93rd Sutherland Highlanders, backed up by Ottoman infantry, probably numbering between 400 and 650 men (remember the regiment would almost certainly have been under-strength) routed a Russian cavalry charge of 2,500 soldiers.
Cavalry began a swift decline in pitched battle after the Crimea, but it was still used heavily after the war, though its role had changed. In the US Civil War it was used primarily for raids on enemy supply lines, flank harassment, reconnaissance, and screening. Confederate partisan (guerrilla) bands on horseback operated behind enemy lines and undermined Union control of captured territory. Nathan Bedford Forrest was a master at this and his cavalry terrorized Union detachments. However, it was used less often in conventional battle, where cavalry units were instead typically used as mounted infantry. Cavalry did play a crucial role at Gettysburg, but more for their repeating rifles than their mobility. The last incident of cavalry being seriously deployed was during the First World War, when the first engagement of the conflict involved a small group of British lancers encountering a group of opposing German cavalry; an inconsequential early victory for the British in what was to become a war characterised by the futility of charging prepared machine gun positions. War, from that point on, was to be decided by the tank, the infantryman, and the artillery shell, and especially the plane, and the era of the cavalryman drew to a permanent close.
That said, The Polish Soviet War and the Russian Civil War before that, had considerable use of cavalry / dragoons. In general eastern Europe was more cavalry country then western, and the age of the cavalry reached its conclusion later there.
All that said, mounted cavalry has continued to be used off-and-on into the 21st century, though later examples end up being cases of Schizo-Tech (the civil war in Afghanistan before the Americans invaded featured images such as men riding horses and carrying AK-47s). If you lack easy access to motor vehicles or aircraft, but you have horses, their mobility advantages still apply. During the early days of World War II, the Poles had an anti-tank rifle that could be transported on a horse and quickly removed and set up for firing. It would be more accurate to say that horses can be used in some modern situations as dragoons rather than cavalry. The difference being that while a Cavalry trooper is trained to fight from horseback, Dragoons are trained to fight on foot, mainly using the horses to quickly get near their objectives (although around the 18th century, the word "Dragoon" evolved to mean "Light Cavalry").
Also, it is a common spelling error to refer to men riding horses in battle as "Calvary". Calvary is a hill in Jerusalem. Easy way to remember the spelling: In the war movies, you always hear them talk about "Armored Cav" or "Airborne Cav", or talk about "Cav Troopers". Never "Cal".
In many mechanized armies, troops who fight in armored vehicles or from helicopters are often referred to Cavalry as well, and consider themselves the spiritual descendants of horse-mounted cavalry.
One thing worth bearing in mind about cavalry is that they are not good on rough or hilly terrain; horses do the most damage when they are charging over flat ground. Charging uphill just tires the horses, and charging downhill risks a fatal stumble. Difficult or broken ground can also throw a tightly-packed heavy cavalry charge into chaos just as well as a pikeline can be thrown out by the same ground. Camels also terrify horses who aren't used to their distinctive odour.
It should also be noted that not all cavalry was based around the horse. Desert regions saw the use of camels, where their slower speed on solid ground was out weighed by their ease at negotiating sandy terrain, and the easy logistics provided by their water hoarding abilities (of critical importance in the desert).
Elephants also saw action, essentially being used as super-heavy cavalry. Despite lacking any real finesse and not being particularly maneuverable, their size allowed them to either house multiple archers as opposed to the horse's one, a single heavy weapons platform (usually a ballista), or alternatively, just trample over the enemy lines. Elephants did however suffer from the disadvantage of turning and fleeing from certain sights (usually incendiary weapons) or upon injury--this is why each war elephant had a man seating on the back of its neck with a hammer and stake to put the beast down should it panic; if this man was incapacitated and elephant somehow spooked, it was not rare for it to make a U turn and trample its own lines. Due to this weakness, Alexander the Great used his Indian elephants for logistics only. In the regions where elephants were available, they eventually fell from grace as gunpowder weapons improved, even before the horse, and they just served as a big target.